The invention relates to an MR imaging method, notably for real-time imaging, wherein temporally successive MR data of an object to be examined is acquired by the repeated application of imaging pulse sequences, said MR data being subjected to phase correction in order to compensate for phase errors.
The invention also relates to an MR apparatus for carrying out the method which includes a gradient coil system for generating magnetic field gradients, an RF coil system for generating RF pulses and for receiving MR signals, a control unit which controls the gradient coils and RF coils so as to apply imaging pulse sequences to an examination zone, and a reconstruction unit whereby MR data is stored and subjected to a phase correction in order to compensate for phase errors.
The invention also relates to a computer program for controlling the MR apparatus.
In recent years numerous technical developments of the hardware and software of MR apparatus were aimed at achieving a drastic reduction of the image acquisition times. The image rate that can be achieved has meanwhile become so high, that real-time MR imaging has become an important tool in various medical applications. Real-time MR imaging is of special interest for diagnosis in the field of cardiology, for example for dynamic examinations of the function of the coronary vessels and the cardiac muscle. Further possible applications can be found in the field of interventional radiology, for example, for monitoring minimal invasive interventions, biopsies etc. that are performed with an MR tomography apparatus. The users of such methods need high quality images with an as high as possible image rate, for example, in order to enable continuous observation of the motions of the cardiac muscle or the interventional instruments, that is, in real time. Therefore, there is serious interest for further improvement and acceleration of the methods for real-time MR imaging.
As is generally known, magnetic resonance tomography is a spectral imaging method wherein a spatially inhomogeneous magnetic field (magnetic field gradients) is used to localize the nuclear magnetization on the basis of the respective associated resonance frequency. It is common practice to record the magnetic resonance signal as a voltage, induced in a coil that encloses the examination zone, under the influence of a suitable sequence of RF pulses and gradient pulses in the time domain. The actual image reconstruction is performed by Fourier transformation of the time signals. The number, the distance in time, the duration and the strength of the gradient pulses and RF pulses used govern the sampling of the reciprocal so-called k space which determines the volume to be imaged and also the image resolution. Similarly, the requirements imposed as regards image format and image resolution determine the number of phase encoding steps required, and hence also the duration of the imaging sequence. The EPI (Echo Planar Imaging) sequence is a customarily used pulse sequence for the sequential sampling of the k space and enables fast imaging and hence is very important for real-time MR imaging. Such an imaging pulse sequence produces a two-dimensional image of a selected slice in the volume to be examined.
An MR imaging method of the kind set forth is disclosed, for example, in U.S. Pat. No. 5,621,321. The MR data sets in the known method are acquired by means of a suitable imaging pulse sequence and subjected to a phase correction in order to compensate for phase errors which would otherwise give rise to undesirable image artefacts. It is a generally known phenomenon that initially the MR signals still suffer from phase errors which are caused partly by so-called eddy currents which are induced in the vicinity of the examination zone by the magnetic fields of the imaging pulse sequences that vary quickly in time. According to the known method a phase correction profile is determined by means of a calibration pulse sequence, said phase correction profile being used to compensate for the phase errors present in the actual MR image data. The phase correction profile consists of a set of complex phase factors which enable a linear and a non-linear phase correction of the overall MR signal.
The eddy current behavior, having a decisive effect on the phase errors of the MR signals, is dependent to a high degree on the direction, the number and the strength of the gradient pulses used and also on the relative position of the object to be examined within the MR system. This means that the phase errors change as soon as, for example, the position and the orientation of the image plane are modified or as soon as the position of the object to be examined changes. Consequently, the phase correction profile is also highly dependent on the selected image plane and in the known method it must be measured again by means of the calibration pulse sequence as soon as the position of the image plane is changed or as soon as the position of the patient in the MR apparatus changes. Moreover, slow variations occur in the eddy current behavior; such variations are caused by thermal effects on the MR apparatus and also necessitate updating of the phase correction profile.
For many real-time MR imaging applications it is necessary to perform interactive adaptation of the position of the image plane during the continuous imaging in order to enable optimum and continuous tracking of given motions, for example, of interventional instruments. In conformity with the known MR imaging method it would then be necessary to measure the phase correction profile all the time again by means of the calibration pulse sequence, thus leading to an undesirable delay of the imaging because a significant part of the overall measuring time is used for the determination of the phase correction profiles.
The eddy current behavior of an MR system is extremely complex and very difficult to predict. Therefore, there is a possibility that given changes of the position of the image plane lead to small changes only of the phase errors. In that case the repeated measurement of the phase correction profiles for each change of the image plane is an unnecessary waste of costly measuring time in given circumstances.
Therefore, it is an object of the present invention to provide an improved MR imaging method in which the image rate is further increased in comparison with the known method in that the measuring time that has to be spent on determining the phase correction profiles is reduced as much as possible.
This object is achieved by means of an MR imaging method of the kind set forth in that the phase errors are continuously monitored in parallel with the application of the imaging pulse sequences in that MR data acquired at different instants are related to one another.
The invention is based on the idea to use the MR data sets acquired for imaging also for the purpose of monitoring the behavior of the phase errors continuously, without it being necessary to perform time and again time-consuming calibration measurements so as to obtain the phase correction profiles. Changes in the eddy current behavior, caused by changes of the image plane, by motions of the object to be examined or by thermal drift, can be recorded on the basis of the phase errors when at least two MR data sets that have been acquired at different instants are related to one another. It is notably when a series of EPI sequences is used for imaging that changes in the eddy current behavior can be detected already on the basis of two individual echo signals that have been acquired separately in time.
For the monitoring of the phase errors in accordance with the invention, MR data acquired with the same phase encoding and with oppositely directed but equal read-out gradients are related to one another. The phase of the acquired MR signals is influenced not only by the described eddy currents, but also by inhomogeneities of the steady magnetic field and by the chemical shift of the excited nuclear magnetization. These different components must be separated during the phase correction so that, if possible, only the phase errors that are due to the eddy currents are compensated. This is achieved by determining the phase differences between MR signals, such as single echo signals in a series of EPI sequences, that have been acquired with the same phase encoding and with respective opposed read-out gradients. The phase differences thus obtained are based essentially on the effects of the eddy currents, so that continuous monitoring of changes in the eddy current behavior can take place on the basis of such phase differences.
MR signals acquired with the same phase encoding and with opposed, equal read-out gradients can be advantageously combined so as to calculate a phase correction profile. This is because a phase correction profile can be generated from the phase differences between these signals; this phase correction profile can be used to compensate for the phase errors of the MR data. The distance in time between the acquisition of the MR data thus combined preferably is not very large, so that the instantaneous phase errors can be correctly determined and corrected on the basis of the phase differences thus determined.
As soon as a significant change of the phase errors is recorded in accordance with the invention, a calibration pulse sequence can be initiated in order to measure a phase correction profile. In comparison with the known MR imaging method this offers the advantage that a calibration measurement is automatically performed only if it is really necessary, that is, when the eddy current behavior has changed to such an extent that the relevant phase correction profile used is no longer valid. The measuring time required for the phase correction by way of the additional calibration pulse sequence is thus minimized. During the continuous monitoring of the phase errors in accordance with the invention a statistical analysis of the phase difference profiles determined can be carried out; for these profiles a threshold value can be preset in order to evaluate the significance of the changes in the eddy current behavior on the basis thereof.
For the MR imaging method in accordance with the invention it is advantageous to change the sign of the read-out gradients in the successively applied imaging pulse sequences. This is because alternately MR signals, such as individual echo signals of an EPI sequence, are then picked up with the same phase encoding but with opposed read-out gradients. This offers the advantage that in accordance with the invention such directly successively acquired MR data can be related to one another in order to monitor the phase errors but can also be combined directly in order to determine an instantaneous phase correction profile.
For real-time MR imaging it may be advantageous to use imaging pulse sequences with interleaved phase encoding steps (a so-called sliding window reconstruction). This offers inter alia the advantage that multiple image reconstruction can take place during a single complete sampling of the k space, thus yielding a higher image rate (see S. Riederer et al., Magnetic Resonance in Medicine, Vol. 8, pp. -15, 1988). If the phase errors are to be monitored in accordance with the invention during such an imaging method, the sign of the read-out gradients is preferably changed after every complete sampling of the k space. This offers the advantage that the phase differences of successively acquired MR signals can be related to one another with each time the same phase encoding but with opposed, equal read-out gradients, as described above, in order to monitor the phase errors or to determine phase correction profiles.
Imaging pulse sequences such as the already mentioned EPI (Echo Planar Imaging) or the EVI (Echo Voluminar Imaging) sequences are particularly suitable for real-time MR imaging. Such ultrafast imaging sequences, in which the read-out gradient is continuously switched to and fro in order to generate gradients, are capable of generating a complete image by means of only a single RF excitation. The acquisition times for a single image are then substantially less than 100 ms. In accordance with the invention the EPI and EVI sequences can be applied in such a manner that individual echo signals of successive acquisitions can be related to one another for continuous monitoring of the phase errors. As described above, this operation is advantageously performed in such a manner that individual echo signals with the same phase encoding and an opposed, equal frequency encoding are combined so as to determine the phase differences. To this end, simply the signs of the read-out gradients of temporally successive EPI or EVI sequences can be periodically changed or the k space can be sampled in a different manner such that MR signals are acquired with the same phase encoding and each time alternating read-out gradients. The monitoring of the phase errors in accordance with the invention then takes place directly on the basis of the image data, without valuable measuring time being wasted on calibration sequences. It may even be that calibration measurements can be completely dispensed with, that is, when the phase correction profiles are also calculated directly (as described above) from the image data.
The MR imaging method in accordance with the invention can be carried out by means of an MR apparatus which includes a gradient coil system for generating magnetic field gradients, an RF coil system for generating RF pulses and for receiving MR signals, a control unit which controls the gradient coils and RF coils so as to apply imaging pulse sequences to an examination zone, and a reconstruction unit which stores MR data which is subjected to a phase correction in order to compensate for phase errors, the reconstruction unit being programmed in such a manner that in parallel with the application of the imaging pulse sequences the phase errors are continuously monitored by relating stored, temporally successively acquired MR data to one another. The method in accordance with the invention can thus be readily implemented in a customary MR tomography apparatus in clinical use, without necessitating any modifications of the hardware. It is merely necessary to adapt the programming of the reconstruction unit. This can be realized by means of a computer program for controlling the MR apparatus which, in order to evaluate phase errors, relates at least two of the MR data sets stored in the reconstruction unit to one another and determines phase differences therefrom. In accordance with the invention such MR data sets are MR image data which is acquired by means of an imaging pulse sequence and evaluated in order to monitor the phase errors. Such a computer program can notably carry out a direct phase correction of the MR data on the basis of the phase differences determined. To this end, the program has to communicate with the control unit so as to relate the MR signals, on the basis of which the phase errors can be monitored, correctly to one another as described above. Such a computer program can be advantageously presented to users of MR apparatus by storing it on a suitable data carrier, such as a disc or a CD-ROM, or by downloading it via a data network.